In the second of a three-part series, Paul Shillito, engineering director at Oliver Valves, looks at how to avoid hydrogen-induced stress cracking in subsea valves.
As we discussed in part one, subsea valves are a critical part of successful offshore drilling and production operations. Yet, corrosion can cause failure which can ultimately result in a failure or malfunction and installing replacements can be a very costly exercise.
Cathodic protection is an essential process in valve protection, but the downside is that it tends to can damage some the valve casing material as a result of penetration by hydrogen ions.
These ions are created at the surface of the metal by the electrolytic process that occurs in the water as a result of the electric potential between the valve and the anode. Most of the hydrogen ions produced will leave the metal as harmless hydrogen gas which will simply bubble up to the surface.
However, a small portion of the ions will penetrate the some metal, making it brittle. While the level of penetration of hydrogen ions may be small, the pressures at which subsea equipment is required to operate can create significant stresses in the material. This can lead to cracks forming in the material – a process known as hydrogen-induced stress cracking (HISC).
This hazardous effect can only be combated by ensuring that the material used to create the valve is resistant to the effects, and that the stresses in the material are not high enough to result in cracking. A recommended practice document has recently been published to outline how HISC can be avoided in duplex steels and draw the industry’s attention to the risks involved.
This document, produced by Det Norsk Veritas (DNV), details factors such as the coarseness of the material’s grain, the acceptable levels of linear elastic stress and non-linear strain – both during manufacture and operation – and the recommended testing procedures to ensure that the design meets these criteria.
In our next and final instalment, I’ll look at the rigorous testing process that each subsea valve needs to go through to ensure it will function correctly once in position on the seabed.